Current vector control for controlling a speed and torque of a motor to a desired value has conventionally been known. Various control methods have been proposed for current vector control, and for example, id=0 control, maximum torque control, field-weakening control, and the like have been known.
Id=0 control refers to a control method for maintaining a d-axis current at 0, and it can readily realize linear control of torque. Maximum torque control refers to a control method for maximizing generated torque with respect to the same current, and it can efficiently generate torque. Field-weakening control refers to a control method for reducing magnetic flux in a d-axis direction by feeding a negative d-axis current, and it can broaden a speed control range.
Japanese Patent Laying-Open No. 10-328952 discloses a control device for a permanent magnet synchronous motor utilizing such current vector control. The motor control device including a speed control unit and a torque control unit broadens a motor speed control range by utilizing equivalent field-weakening control and carries out speed control and torque control while independently maintaining speed control accuracy and torque control accuracy.
Among characteristics required in the motor, quietness (low vibration characteristic and low sound noise characteristic) is also important in addition to speed controllability and torque controllability. In particular, when the motor is mounted on a vehicle, quietness of the motor is highly required in order not to impair comfortability in the vehicle. In the vehicle, however, due to restricted space for mounting the motor, weight reduction or the like, measures against vibration or sound noise supported by hardware cannot sufficiently be taken in many cases.
Under the circumstances, motor control paying attention to quietness has been demanded. Japanese Patent Laying-Open No. 10-328952 above, however, does not particularly take into consideration quietness of the motor, and the control method disclosed in this publication cannot improve quietness of the motor.
In addition, in motor control capable of improving quietness of the motor, motor control stability should not be impaired and approaches for ensuring control stability are also necessary.